This invention relates to a tone production method for an electronic musical instrument and, more particularly, to a method capable of continuously varying partial contents, particularly harmonic overtone components i.e. spectral construction of a tone wave and thereby continuously varying the tone color of the produced tone.
Various methods have been proposed to synthesize musical tones in an electronic musical instrument. One of the proposed methods is a technique disclosed in the specification of U.S. Pat. No. 3,809,786 entitled "Computor Organ" According to this method, Fourier components (harmonic ingredients) of a musical tone are individually computed and summed up to synthesize the musical tone. This method is meritorious in that a wide range of musical tones can be synthesized but is disadvantageous in that it requires a large number of computation circuits resulting in bulkiness in construction of the electronic musical instrument. This prior art method is also accompanied by technical difficulties that increase in the number of harmonics used for synthesizing a musical tone requires expansion of a harmonic coefficient memory for storing correspondingly increased number of harmonic coefficients and also requires an increased frequency of a clock used for computation for shortening time for computing the harmonics. If the number of harmonics is to be increased in the prior art method with the frequency of the computation clock being unchanged, a parallel processing system must be introduced and this requires a further enlargement of construction of the electronic musical instrument.
There is also a prior art method for producing a musical tone utilizing a frequency modulation technique as disclosed in the specification of U.S. Pat. No. 4,018,121. This prior art method has overcome the above described disadvantage of the Fourier components synthesizing method fairly effectively for it can produce many partial tones or harmonic or unharmonic components by calculation of a simple mathematic equation. This prior art method is particularly effective for synthesizing percussion instrument sounds (including piano) and wind instrument sounds. The prior art method, however, is disadvantageous in that the amplitudes of respective partial tones become irregular, i.e., irregularity occurs in the spectrum envelope of the musical tone if a large modulation index (I) is used, so that the method is not very suitable for producing a tone having a relatively smooth spectral construction (e.g. string instrument tones).